In recent years, service conditions for oil well tubes and boiler tubes are getting much more hostile. For this reason, requirements for seamless tubes to be used therefore are becoming more rigorous. For example, oil well tubes used for deeper oil wells and more corrosive environment are required to have higher strength and better corrosion resistance. On the other hand, tubes used in nuclear power generation facilities, chemical plants, and the like are required to be excellent in corrosion resistance, particularly in stress corrosion cracking resistance in high temperature pure water or hot water containing chlorine ions (Cl−). From these requirements, a seamless tube made of a high alloy containing a large amount of Cr and Ni, and also Mo is being applied.
For example, Patent Document 1 discloses a high Cr-high Ni alloy which contains Cr: 20 to 35%, Ni: 25 to 50%, Cu: 0.5 to 8.0%, Mo: 0.01 to 3.0% and sol. Al: 0.01 to 0.3% and in which the contents of Cu and Mo satisfy a relationship represented by: % Cu≧1.2−0.4(% Mo−1.4)2, as a high alloy for seamless tubes having high strength and being excellent in corrosion resistance and hot workability, the seamless tubes being used for deep wells and oil wells or gas wells (hereinafter, simply referred to as “oil wells”) in severe corrosive environments.
As a process for producing seamless tubes, employed are processes in which a billet as being a high-alloy starting material to be extruded is used to make a high-alloy tube applying a hot rolling process such as a hot extrusion tube-making process represented by the Ugine-Sejournet process or the like, and the Mannesmann tube-making process.
FIG. 1 is a sectional view for describing a hot extrusion tube-making process used for producing a seamless tube. A billet 8 with a through hole along the longitudinal centerline (in the present specification, simply referred to as a “hollow billet” or a “billet”) is placed in a container 6, and a die 2 is detachably fitted to one end of the container 6 by the intervention of a die holder 4 and a die backer 5. A mandrel 3 is inserted into the through hole of the billet 8, and a dummy block 7 is arranged on the rear end surface thereof.
In such a configuration, when the dummy block 7 is pressed in the direction of a white arrow by actuating a stem which is not shown, the hollow billet 8 is upset and then extruded from the annular space formed by the inner surface of the die 2 and the outer surface of the mandrel 3, producing a seamless tube having an outside diameter corresponding to the inside diameter of the die 2 and an inside diameter corresponding to the outside diameter of the mandrel 3. In the production of the seamless tube, a hollow glass disk lubricant 1 is placed between the die 2 and the hollow billet 8 in order to lubricate between the inner surface of the die 2 and the front end surface and the outer surface of the hollow billet 8.
In addition to the Patent Document 1, the prior art in which a hot extrusion process is applied to the production of high-alloy tubes includes the following. Patent Document 2 describes that a billet made of an alloy in which the contents of Cr, Mo, W and the like are specified has been subjected to hot extrusion processing to form a blank tube having an outside diameter of 60 mm and a wall thickness of 4 mm, which has been then subjected to heat treatment and cold working to produce, for test evaluation, an alloy tube excellent in stress corrosion cracking resistance. Patent Document 3 describes that an alloy in which the contents of Cr, Ni, Mo, Al, Ca, S, O, and the like are specified has been subjected to a hot extrusion tube-making process to produce a blank tube. The Patent Document 1 also describes that the billet made of the above high Cr-high Ni alloy has been used to form a tube having a diameter of 60 mm and a wall thickness of 5 mm by hot extrusion tube-making represented by the Ugine-Sejournet process.
However, the Patent Documents as described above only disclose that hot extrusion has been performed, and no document discloses the findings in which processing-incurred heat, occurring during hot extrusion of an alloy having a high deformation resistance, is taken into consideration, with respect to the suppression of cracking and/or seam flaws incurred by grain boundary melting.